Copper interconnect devices are gaining rapid acceptance over aluminum interconnects, due to superior electrical conductivity of copper. Practicing what is known in the industry as damascene technology, one typically deals with interconnect trenches or vias that are filled with electroplated copper.
Ideally, copper electroplating would fill the trenches to be “level” with the patterned barrier layer underneath said copper “filling”, or even more desirably, electroplated copper would fill said trenches only to the level of the surface of the dielectric. In actual industrial practice however, electroplated copper “overfills” and protrudes said trenches, resulting in a rough, “bumpy” surface topography, as opposed to one that is smooth or “planar”. Thus, the pursuit of planarity has become a top priority in the fabrication of ULSI devices.
Chemical Mechanical Planarization (CMP) is currently the industry's premier choice to achieve planarization, as it serves to remove “excess”, unwanted material that interferes with planarity.
CMP is perceived to be a complex process, involving both mechanical grinding or abrading, and also chemical etching/oxidation. The detailed, scientific mechanism of CMP is yet to be elucidated, especially as it relates to its chemical component. Hence, CMP still remains a very complex operation, relying principally on empirical, trial-and-error developments.
While great strides are being made to improve CMP processes (as reflected in the numerous patents which suggest approaches/solutions to given tasks), planarity problems associated with copper still remain perhaps the industry's greatest challenge, yet to be resolved satisfactorily.
The problem relating to planarity of copper conductors, is known in the trade as “dishing”, which can be somewhat simplistically described as denoting the undesirable reduction of thickness of the metal line, causing an increase in electrical resistance and higher current density at a given voltage, than what the conductor has been designed for, resulting in functional deficiency of the interconnect device.
Even though the precise causes and mechanisms of dishing are poorly understood, as noted previously, there appears strong indication that dishing is caused both by mechanical gouging, scratching, overpolishing by the polishing media, as well as uncontrolled, indiscriminate chemical erosion of copper.
In embodying CMP processes and compositions, the prior art predominantly uses aqueous solutions or dispersions principally consisting of slurries of abrasive particles that comprise oxidizers, surfactants, viscosity modifiers, and the like. Abrasive particles are delivered to the wafer substrate via a porous polymer polishing pad, which polymer pad, embedded with the abrasive particles, is rotated against the wafer substrate to be planarized, where it removes unwanted, protruding matter. More recent developments disclose fixed abrasive pads that obviate the use of abrasive slurries, calling only for “cooling fluid” during CMP.
The inherent difficulty with above liquid systems, wherein water is essentially the principal, often sole, vehicle that carries the slurry to areas to be planarized, is that the distribution of abrasive particles in such aqueous, liquid media is non-uniform, resulting in uneven polishing, that aggravates dishing.
There is therefore a need in the industry for improved CMP compositions and methods that alleviate problems of planar non-uniformity, especially as they relate to dishing. There is also a need for an additional/alternate vehicle, other than aqueous suspensions that are prone to compositional non-uniformity, to deliver CMP ingredients/compositions to the wafer substrate.